In induction machines the current in the secondary winding (usually the rotor) are created solely by induction. These currents result from voltages induced in the secondary windings by rotating magnetic fields in the primary winding which arise from the application of line voltages thereto.
When the machines are operated as a motor, the rotor rotates at speeds below the synchronous speed. The difference in speed is referred to as the slip speed, usually expressed as a decimal of the synchronous speed.
The rotor windings are generally connected to slip rings and adjustable resistances are connected in series with the windings. The resistances limit the secondary currents during "start." As the motor picks up speed the secondary resistance is gradually reduced whereby the efficiency increases. The resistance can also be used to control the speed; however, this method of speed control is very inefficient.
When the rotor is driven at above the synchronous speed, the machine acts as a generator. With resistance in the secondary windings, the output power can be maintained somewhat constant over a narrow range of rotor speeds.
For the past thirty years or more when driving a generator with various types of prime movers, the speed of the electrical generator was kept nearly constant. Various mechanical methods for controlling speed have been used depending on the prime mover. When using an alternator with DC excited fields, the rpm must be kept constant to a very close tolerance; less than one revolution of 1800 or 3600 revolutions per minute. When using a squirrel-cage induction generator, the most common cogeneration generator, a few percent above base rpm is necessary. If inadvertently, a higher speed is supplied by the prime mover, the generator completely releases its load and a "runaway condition" exists. Under such circumstances, the prime mover, a wind or steam turbine, or a diesel, may race to destruction in a very few minutes or seconds.
In equipment for wind, water, wave power, thermal, etc., mechanical means are used to maintain constant speed. Spoilers, blade pitch control and mechanical brakes are used to limit the speed. All these methods have a very short life and are costly to maintain.
In U.S. Pat. No. 2,648,808 there is described a motor having a wound primary winding (stator) in which the effective impedance of the primary windings is varied to improve the torque-speed characteristics of the motor. More particularly, the power factor of the motor is improved by controlling the impedance of the primary windings by adding thereto external series impedances.